In recent years, the popularity of systems using wireless radio communication has increased substantially. For example, cellular communication systems and wireless networks have now become commonplace. The increased requirement for frequency spectrum resource has led to an increased desire for efficient communication and especially at higher frequencies and for higher data rates.
For example, Wireless Local Area Networks WLANs are becoming common not only in business environments but also in domestic environments. In order to increase the capacity of such WLANs, it is desirable to increase the data rate of the wireless communication. As a specific example, the Institute of Electrical and Electronic Engineers (IEEE) have formed a committee for standardising a high-speed WLAN standard known as IEEE 802.11n™. The 802.11n™ standard comprises Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications aimed at enhancing WLANs to provide higher effective data throughputs. It is intended that the 802.11n™ standard will help WLANs meet the expanding bandwidth needs of enterprise and home networks, as well as those of WLAN hot spots.
In order to achieve high data rates over the air interface, a number of advanced radio techniques are employed. It has been found that in systems using open-loop approaches (i.e. without the transmitter using knowledge of the transmit channel or the signal received at the receiver) significant improvement can be achieved by using multiple antennas at the transmitter and the receiver. In particular, many radio communication systems, such as WLANs, provide for a plurality of transmit and receive antennas to be used. Specifically, some transmission techniques involve transmitting a data stream by simultaneously transmitting different signals derived from the data stream from different antennas over the same communication channel. The receiver(s) of these techniques typically also comprise a plurality of antennas each of which receive a combined signal corresponding to the transmitted signals modified by the individual propagation characteristics of the radio link between the individual antennas. The receiver may then retrieve the transmitted data stream by evaluating the received combined signal.
Such techniques are known as Multiple Transmit Multiple Receive (MTMR) or Multiple Input Multiple Output (MIMO) schemes and can be designed to derive benefit from spatial diversity between the antennas in order to improve detection. Indeed, the equivalent Signal to Noise Ratio (SNR) of the combined signal is typically increased compared to the single antenna case thereby allowing higher channel symbol rates or higher order constellations. This may increase the data rate for the communication link and thus the capacity of the communication system.
For the situation where two transmit antennas are used, an efficient transmit case has been proposed in “A simple transmit diversity technique for wireless communications,” by S. M. Alamouti IEEE Journal on Selected Areas in Comm., pp. 1451-1458, October 1998. The proposed technique is known as Alamouti coding and comprises transmitting two symbols from two antennas during two symbol times such that the original symbols may easily be derived by a receiver. The proposed technique is a specific case of Space-Time Block Coding (STBC) and results in high performance for two transmit antennas by providing for orthogonal transmission of the two symbols while fully exploiting the spatial diversity of the two antennas.
In order to increase the spectral efficiency of the system it has been proposed to extend the Alamouti coding technique to more than two transmit antennas by simultaneously transmitting different data streams from different antenna pairs. For example, a single data stream may be split into two sub-data streams which are then individually Alamouti coded and simultaneously transmitted over the same communication channel from different antenna pairs. Thus, in this technique, data is transmitted over four transmit antennas by individually Alamouti coding two data streams and simultaneously transmitting each of the resulting data streams over two of the four antennas. Hence, two data streams are transmitted simultaneously from four antennas, each stream being encoded using the Alamouti code. However, although improved performance may be achieved over a classical four antenna space division multiplexing scheme, the resulting data rate is significantly reduced due to the symbol doubling associated with the Alamouti code.
The increased spectral efficiency of STBC is due to the exploitation of spatial diversity and specifically is due to the system reducing the probability of deep fades by averaging over different transmit branches. However, as a result, the combination of STBC with other techniques that tend to exploit variations in the experienced communication channel tend not to provide the expected performance benefit. For example, in the article “The impact of multiuser diversity on space-time block coding”, Ran Gozali, R. Michael Buehrer, and Brian D. Woerner, IEEE Communication Letters, 7(5):213-215, May 2003, it is demonstrated that the performance gain of using STBC in a multi-user context is relatively limited. Indeed the application of STBC results in a suppression of deep fades but also results in peaks being suppressed resulting in an averaging of the experienced SNR. As a consequence, scheduling systems that are aimed at scheduling users when they experience advantageous propagation conditions will tend to be less efficient as the peak conditions during which users are typically scheduled are reduced by the STBC technique.
Furthermore, the resource and computational requirements associated with STBC communication and scheduling tends to be substantial resulting in increased cost, resource usage and complexity.
Hence, an improved communication system would be advantageous and in particular a system allowing increased flexibility, facilitated implementation, reduced complexity, reduced resource usage, increased spectral efficiency, improved and/or facilitated multi-user operation and/or improved performance would be advantageous.